This nonprovisional application claims priority under 35 U.S.C. xc2xa7 119(a) on Patent Application No. 2001-219318 filed in Japan on Jul. 19, 2001, the entirety of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an output shaft structure of a personal watercraft.
2. Description of Background Art
Two-cycle engines have been used as power sources of general personal watercrafts. In recent years, however, to meet a requirement to lower environmental pollution and reduce noise, personal watercrafts using four-cycle engines as power sources have been proposed (see Japanese Patent No. 2880691).
Since integral type crankshafts are generally used for four-cycle engines, plain metal is used as bearings for the crankshafts.
In personal watercrafts of this type, as shown in FIGS. 10(a) and 10(b) of the present invention, an engine 1 is mounted on a watercraft body 2 with a crankshaft 1a of the engine 1 extending along the longitudinal direction of the watercraft body 2. A shaft 3a of a propulsion device (for example, a jet pump) 3 is coupled via a coupling 4 to a rear end 1b of the crankshaft 1a in such a manner as to be disposed on an extension of the crankshaft 1a. 
Accordingly, power from the crankshaft of the engine 1 is transmitted to the propulsion device via the shaft 3a of the propulsion device 3, to propel the watercraft body 2.
In the personal watercraft as shown in FIGS. 10(a) and 10(b), a reaction force from the propulsion device 3 is transmitted to the crankshaft 1a of the engine 1.
To be more specific, the crankshaft la receives an anti-torque, a thrust force, a bending force (for deflecting the crankshaft), and vibration from the shaft 3a of the propulsion device 3 via the coupling 4. These forces and vibration are applied in a complex combination to the crankshaft 1a. 
In the conventional personal watercraft as shown in FIGS. 10(a) and 10(b), if a two-cycle engine is used as the power source, there occurs no problem. This is because a crankshaft of the two-cycle engine is of a built-up type and uses a ball bearing having high durability as a bearing therefor.
In the conventional personal watercraft as shown in FIGS. 10(a) and 10(b); however, if a four-cycle engine is used as the power source, there occurs a problem. The reason for this is that since a crankshaft of the four-cycle engine is of an integral type and plain metal is used as a bearing therefor as described above, the above-described anti-torque, thrust force, bending force, and vibration are applied in a complex combination to the crankshaft. Accordingly, the crankshaft rotating at a high speed causes torsion, compression, bending (deflection), and vibration in complex combination. This results in the fatigue of the plain metal becoming severe, to degrade durability of the engine.
An object of the present invention is to solve the above-described problem and to provide an output shaft structure of a personal watercraft, which is capable of enhancing durability of an engine.
To achieve the above object, according to a first aspect of the present invention, there is provided an output shaft structure of a personal watercraft, wherein an engine is mounted on a watercraft body with a crankshaft of the engine extending in the longitudinal direction of the watercraft body. A shaft of a propulsion device is coupled to a rear end of the crankshaft of the engine in such a manner as to be disposed on an extension of the crankshaft. The crankshaft is supported by an engine case via plain metal. This output shaft structure includes an output shaft, which is provided separately from the crankshaft and is coupled to the rear end of the crankshaft. The shaft of the propulsion device is coupled to a rear end of the output shaft. The output shaft is supported by the engine case via a rolling bearing, which is immovable in a thrust direction.
With the above construction according to the first aspect of the present invention, a thrust force from the shaft of the propulsion device is mainly received by the output shaft and the rolling bearing. Therefore, the thrust force is not directly applied to the crankshaft. However, if the thrust force is applied to the crankshaft, it is transmitted thereto indirectly (in a moderated state).
Since the output shaft provided separately from the crankshaft is coupled to the rear end of the crankshaft and the shaft of the propulsion device is coupled to the rear end of the output shaft, an anti-torque, a bending force, and vibration from the shaft of the propulsion device are not directly applied to the crankshaft. However, if applied to the crankshaft, they are transmitted thereto indirectly (in a moderated state). In particular, a bending force applied to the crankshaft is significantly reduced.
As a result, according to the output shaft structure of a personal watercraft according to the first aspect of the present invention, it is possible to reduce fatigue of the plain metal for supporting the crankshaft, and hence to improve the durability of the engine.
According to a second aspect of the present invention, a spline is formed on or in the rear end of the crankshaft. Furthermore, a spline is formed on or in the front end of the output shaft. A spline for engageing with the splines is formed in or on an inner surface of a connection pipe. The rear end of the crankshaft is coupled to the front end of the output shaft via the connecting pipe.
With the above construction according to the second aspect of the present invention, it is possible to certainly transmit power from the crankshaft to the output shaft via the connection pipe.
An anti-torque, a bending force, and vibration from the shaft of the propulsion device are transmitted from the output shaft to the crankshaft via the connection pipe. In this case, since each of the crankshaft and the output shaft is coupled to the connection pipe by a spline-fitting, the reaction force, particularly, the bending force and vibration can be significantly reduced in the course of transmission from the output shaft to the crankshaft via the connection pipe.
As a result, according to the output shaft structure of a personal watercraft according to the second aspect of the present invention, it is possible to certainly transmit a power from the crankshaft to the output shaft via the connection pipe and to improve durability of the engine.
According to a third aspect of the present invention, a gap is formed between opposed faces of the rear end of the crankshaft and the front end of the output shaft.
With the above construction according to the third aspect of the present invention, an anti-torque, a thrust force, a bending force, and vibration, particularly, a thrust force and a bending force can be significantly reduced. Accordingly, it is possible to more certainly improve the durability of the engine.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.